STOCKHOLM (Reuters) - Sweden's Left Party warned Social Democrat leader Stefan Lofven on Wednesday it could not back him as prime minister if he accepted demands from rivals in parliament to swing sharply to the right.

Parliament will vote next week on letting Lofven form a new government and he needs support from parties that are polar opposites in terms of ideology.

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A September election gave neither the centre-left or the centre-right a majority, leaving the balance of power with the Sweden Democrats, a hard-right, anti-immigration party that mainstream parties refuse to deal with.

This week, the Centre Party - a member of the centre-right Alliance - said it could back Lofven if he adopted policies such as lower taxes, less restrictive labour laws and relaxed rent controls, and excluded the Left Party from having a say.

The Left Party, a long-time partner of the Social Democrats, said on Wednesday, it would not accept that.

"The Left Party is totally uninterested in a government that causes massive rent hikes and forces people from their homes and, with government intervention, lower wages in the Swedish labour market," Left Party Leader Jonas Sjostedt told reporters.

Magnus Hagevi, political science professor at Linnaeus University, said Lofven would find it hard to accept the Centre Party's demands.

"Even if he did, it is not certain he would get the support of the majority of his own party for them in parliament," Hagevi said.

If Lofven loses the vote, Sweden will move closer to snap elections. Ulf Kristersson, leader of the centre-right Moderates, has already lost one vote and elections would be triggered automatically after four tries.